1. Field of the Invention
The present invention relates to an image forming apparatus provided with a sheet curl correcting mechanism.
2. Description of the Related Art
An electrophotographic image forming apparatus is constructed in such a manner that: an electrostatic latent image is formed on the surface of a photosensitive drum by light irradiation from an exposure device based on predetermined image information; a toner image is formed on the surface of the photosensitive drum by supplying toner particles onto the electrostatic latent image by a developing device; and the toner image is transferred onto a sheet. Then, the toner image transferred to the sheet is fixed by a fixing device, followed by discharge of the sheet carrying the fixed toner image.
The fixing device includes a fixing roller internally provided with a heater for heating the fixing roller, and a pressure roller disposed as opposed to the fixing roller, with its surface being contacted with the surface of the fixing roller. The sheet after the transfer operation is subjected to a fixing operation by being heated by the fixing roller, while passing through a nip portion between the rotating fixing roller and the pressure roller.
When the sheet passes through the nip portion, one surface of the sheet having the transferred toner image is heated by the fixing roller, and the other surface of the sheet is contacted with the pressure roller whose surface temperature is lower than that of the fixing roller. As a result, the one surface of the sheet is subjected to thermal expansion. Thereby, widthwise ends of the sheet discharged from the fixing device may be warped toward the pressure roller, in other words, a curl may be formed on the sheet. When the curl is formed on the sheet, the entire thickness of the sheet may be unduly increased, with the result that the discharge tray may be incapable of holding a predetermined amount of sheets.
In order to solve the above drawback, Japanese Unexamined Patent Publication No. 9-240901 discloses a curl correcting mechanism including a curl amount detecting sensor for detecting the curl amount of a sheet discharged on a discharge tray, and a decurler for correcting the curl. When the curl amount detecting sensor detects a curl of a sheet, the decurler is activated to correct the curl of the sheet, and thereafter, the sheet with its curl being corrected is discharged onto the discharge tray. The curl amount detecting sensor continues to detect the curl amount of a sheet to be successively discharged onto the discharge tray, and judges whether a further sheet is allowed to be discharged onto the discharge tray. If it is judged that no more sheet discharging operation is allowed, the sheet discharging operation is suspended.
The curl correcting mechanism recited in the above publication has some drawbacks. Firstly, the decurler is provided upstream of the discharge tray. Specifically, even if a curl of a sheet is corrected by the decurler, the sheet may return to a curled state while being transported along a transport path toward the discharge tray. As a result, the sheet whose curl correction is incomplete may be discharged on the discharge tray.
Secondly, the curl correcting mechanism may be costly, which may resultantly increase the production cost of the image forming apparatus. The decurler generally includes a pair of rollers, a belt wound between the roller pair, and a correction roller biased against a forward portion of the belt.
Thirdly, the curl correcting mechanism is constructed in such a manner that the sheet discharging operation is suspended based on the curl amount of the sheet detected on the discharge tray. In this arrangement, the sheet discharging operation may be suspended despite that a further sheet discharging operation is possible under an actually small curl amount of the sheet on the discharge tray. As result, the sheet processing performance of the image forming apparatus may be degraded.